The Development of Scyphomedusse. 235 



the outer or ectoderm cells, entirely obliterating the cleavage 

 cavity. 



While more recent investigations have tended to discount the 

 older views as to the importance attaching to the methods of for- 

 mation of the primary germ layers, it will not be without some in- 

 terest to briefly refer to some phases of the process. In addi- 

 tion to the invagination of cells there may be some immigration, 

 though this is not common and may take place before or after 

 invagination. Such cells appear to take no part in entoderm for- 

 mation, and while their ultimate fate was not determined, they 

 probably degenerate as was found by Glaus (1891), Smith (1891) 

 and Hein (1900, 1903). Some sections seemed to show that a 

 delamination of the cells of the blastula might occur, the radially 

 placed spindle of figure 15 clearly foreshadowing such a process. 

 This is not at all a common condition and so far as could be deter- 

 mined the delaminated cells did not take part in the formation of 

 the entoderm. 



The nuclear spindles in cleavage stages are quite similar to regu- 

 lar spindles in other forms, having very clear, though delicate, 

 spindle fibres with asters at either pole ; in no case was it possible 

 to demonstrate the presence of any centrosphere or centrosome. 

 The chromosomes in cleavage spindles are well separated, but 

 extremely small. In no instance was there found an amitotic 

 division during the cleavage. 



One of the most characteristic of nuclear phenomena in the 

 cleavage stages of Cynea was found in the ''resting" nucleus. In 

 the majority of eggs, in early cleavage stages, the nuclei were com- 

 posed of several vesicles (figs. 12-14, 20-24). Usually there 

 were two of these vesicles almost i'dentical in appearance and of 

 equal size; other nuclei were single or composed of three to five 

 vesicles, sometimes quite unequal in size (figs. 22, 24). These ves- 

 icles were found rather commonly in the cells of all eggs from the 

 2-cell stage to a blastula containing 24 to 48 cells, though not all 

 the cells in any one egg contained the several vesicles. In tne 

 blastula just before invagination, and in the gastrula, they were 

 less abundant and often entirely lacking, especially in the gastrula. 

 Conklin (1908) in Linerges, sometimes found two equal chromo- 



